1. Field of the Invention
This invention relates to a semiconductor avalanche photodiode (hereinafter referred to as the APD).
2. Description of the Prior Art
The APD is a photo detector having a signal amplifying function and has excellent properties of small size, high sensitivity and high-speed response. In the APD, however, the noise characteristic in an avalanche multiplying region where avalanche gain is attained is not sufficient for an arbitrary wavelength. Especially, a high sensitivity APD operating in a wavelength region longer than 1 .mu.m poses many problems in terms of dark current and noise. The cause of this is considered to reside in that the prior art is based on the concept that an avalanche multiplying region (hereinafter also referred to as an "avalanche region") and a light absorbing region (that is, a photoelectric conversion region) of the APD are formed of the same material, which makes the selection of material very difficult. In a case where the APD is formed of a single substance, for example, Si or Ge, Si does not present serious problems with respect to the noise characteristics and the dark current, but the light absorbing characteristic of Si limits the receiving light wavelength to less than 1.1 .mu.m, while on the other hand Ge is sensitive to a light of the wavelength up to 1.6 .mu.m but has a defect such that an avalanche noise and the dark current are large. Accordingly, in a case of using wavelengths in the vicinity of 1.3 .mu.m, which is regarded as promising in an optical fiber transmisson system, even if use is made of Si, Ge or some other materials, the APD made of a single material is difficult to simultaneously satisfy the requirements of high sensitivity, low noise, low dark current and so forth.